1. Field of the Invention
The present invention refers to a printing device for rotogravure printing machines, and in particular to a printing device comprising an improved doctor member.
2. Background Information
The current rotogravure printing machines are equipped, as can be seen in FIG. 1, with a printing device 1 that substantially comprises at least one printing cylinder 3 equipped on its external surface with a plurality of notches 5 adapted to contain printing liquid (usually ink), and at least one scraper member 7 for every printing cylinder 3. This scraper member 7 is of a type commonly known as doctor and is thrust in contact with the surface of the printing cylinder 3 in order to remove the ink that can be found on the cylinder 3 surface outside of the notches 5, in order to clean the cylinder 3 itself. Ink is xe2x80x9cfishedxe2x80x9d from a tank (not shown) placed under the cylinder 3 and wets the cylinder 3 surface when this latter one performs a rotation printing pass.
Printing on paper supports 2 (usually continuous webs of paper of various types) is carried out through the pressure of a rubber roller 4 that thrusts the paper itself against the printing cylinder 3. This pressure, that is usually applied vertically towards the ground, for 3680-mm wide paper webs (that are the typical paper webs that are commonly used) can reach values that are equal to 15 kg/cm and therefore the rubber roller 4 applies to the printing cylinder 3 a force that is equal to about 6000 kg. Since the printing cylinder 3 is connected to two fixed ends 11, 13 in order to rotate around its longitudinal axis, this force tends to thrust the cylinder 3 downwards (towards the ground) progressively distorting through flexure the cylinder 3 surface up to a value that, next to the central part of the cylinder 3 itself, gets to 1 mm of downwards flexure (starting from a 0-mm flexure value next to the fixed ends 11, 13).
When working, therefore, the printing cylinder 3, the paper web 2 and the rubber roller 4 assume an arc-shaped configuration due to the downwards flexure of all three components, with a flexure difference that progressively goes from 0 to 1 mm along the whole extension of the cylinder 3. Obviously, for different types of workings, different flexure extensions can and could be provided, that anyway all imply the same problem that follows.
The doctor-type scraper member 7 is composed of an elongated blade 16, commonly 0.15-mm thick, that is pressed in contact with the cylinder 3 surface in order to allow it to scrape ink away. Due to the simultaneous presence of a high working speed, of the contact pressure between scraper member 7 and cylinder 3, for the thrust of all ink to be moved and above all for the above flexure to which the cylinder 3 is subjected, the scraper member 7 works differently along the cylinder 3 extension: in fact, ink is completely removed next to the ends 11, 13 and always more poorly removed when going away from the ends 11, 13, since the cylinder 3 surface is also moving away by flexure from the blade 16 of the scraper member 7. This different ink-scraping capability is further translated into a different wear of the scraper member 7, since a smooth contact between itself and the cylinder 3 is progressively missing.
Object of the present invention is solving the above prior-art problems, by providing a printing device 1 in which the scraper member 7 follows with a maximum accuracy the progressive flexure of the printing cylinder 3, in order to guarantee a smooth ink scraping in all its points.
A further object of the present invention is providing a printing device 1 of the above-mentioned type that can be realised with a minimum number of pieces, at a reduced cost and without increasing the final encumbrances of the rotogravure machine to which it is applied.
A further object of the present invention is providing a printing device 1 and a related process for which the force applied to the scraper member 7 in order to allow it to follow the flexure of the printing cylinder 3 can be automatically determined according to the diameter of cylinder 3 and roller 4, and according to other working parameters, also automatically determined.
The above and other objects and advantages of the invention, as will appear from the following description, are obtained by a printing device and a process as claimed in claims 1 and 6. Preferred embodiments and non-trivial variations of the present invention are claimed in the dependent claims.